Microsoft (NSDQ: MSFT) has reached an important milestone in the development of the first major update to Windows Phone 7. It announced Monday that the code has been released to manufacturing, which means that partners–including one very important one in Finland–can start getting their phones ready to ship with the Mango update.

Mango was first previewed back in May, and when it ships later this fall it will be the release that is used on the first Nokia Windows Phone 7 devices. Microsoft’s Terry Myerson, corporate vice president for Windows Phone engineering, said that Mango is ready for “our handset and mobile operator partners to optimize Mango for their specific phone and network configurations” in a blog post highlighting the announcement.

With new features like multitasking, an upgraded browser, and a redesigned communications system, Mango is an important release for Microsoft’s hopes of building Windows Phone 7 momentum. Critics seem to like the software, but it barely registers on market share surveys and among consumers.

Nokia’s hardware could change that equation, especially in Europe, but Microsoft still has a long way to go before its mobile software is considered as much of a force in this industry as iOS or Android. Failing to get updated software out on time, however, would derail any progress, making the release-to-manufacturing milestone quite significant.